Modulated structures in some copper–nickel–iron alloys

Abstract

Some copper-nickel-iron alloys, of compositions characterized by the occurrence of a modulated structure as a pre-preeipitation stage in the transformation of the single-pha~se cubic to the twophase cubic structure, have been re-examined by means of a refined technique which permits X-ray diffraction patterns of powder specimens to be made using only the Ka 1 characteristic radiation. X-ray patterns of single crystals and microscopic examination of lump specimens have also been employed.The experimental results are described and compared with the theory of diffraction for a modulated lattice, to which some extensions are made. The modulated structure is considered to be composed of a regular arrangement of coherent lamellae of two intermediate tetragonal phases, and the wave form of the modulation assumed is rectangular. Excellent agreement between theory and experiment is obtained over the range of existence of the modulated structure.The relation between the diffraction patterns of the modulated structures and those which arise from independent particles of the intermediate phases is discussed, and the possible relevance of this relation to the interpretation of the diffraction patterns of alloys of the age-hardening type is indicated. IntroductionThis paper is concerned with alloys lying in the field marked 'metastable states' in the phase diagram (due to Bradley, Cox & Goldschmidt, 1941) for slowly cooled Cu-Ni-Fe alloys (Fig. 1). This field lies at the extremity * Nowat Division of Tribophysics, Commonwealth Scientific and Industrial Research Organization, University of Melbourne, Australia.of the solid solubility gap in the face-centred cubic solid solution, and in it the tie lines correspond to approximately constant Ni:Fe ratios, so that the separation of the two equilibrium (face-centred cubic) phases involves, chiefly, the diffusion of copper atoms.Bradley (1940) showed that during the annealing of quenched single-phase alloys in this field, two intermediate tetragonal phases appeared--prior to the 302 separation of the cubic equilibrium phases--both having the same a dimension as the quenched alloy, but with axial ratios c/a slightly greater than, and slightly less than, unity. It was assumed that the two tetragonal phases are conjugate, one Cu-rich (c/a> 1), the other Cu-poor (c/a < 1), corresponding to the conjugate cubic phases of the equilibrium state. The powder patterns of alloys annealed for periods too short to produce the tetragonM phases showed 'side-bands' accompanying the lines of the singlephase structure, and Daniel & Lipson (1943, 1944 concluded that the anomMous diffraction effects could be accounted for on the assumption that the lattice parameter is modulated in the directions of the cube axes. The Observed positions of the side-bands were in excellent agreement with the theory of the modulated structure, but the observed intensities of the side-bands, relative to those of the corresponding main lines, could not be explained without the application of a large and somewhat arbitrary ...

Structure and properties of spinodally decomposed Cu-Ni-Fe alloys

“…- ' The side band phenomenon in Cu.,.Ni-Fe alloy~ was discovered by Daniel and Lipso~ (6) and was the subject of later : X '-ray investigations by Hargreaves', (7) Bie~ermannand Kneller (8) The inicrostructure, of the alloy at any stage of its decomposition can be-defined by three m~in parameters:…”

Interpretation of X-ray scattering patterns due to periodic structural fluctuations. I. The case of transverse modulation of positional parameters in primitive lattices

“…Daniel & Lipson (1943, 1944 calculated the scattered amplitudes for the two types of lattice modulations: a sinusoidal modulation of scattering power and a sinusoidal modulation of spacing, which in our terminology is a longitudinal modulation of positional parameters. Hargreaves (1951) tried to introduce a square-wave modulation of positional parameters; however, because of the approximations used he ob-…”

The study of incommensurate structures as a probe to reveal atomic interactions in crystals

“…the three coefficients associated to the three dimensional scattering vector. The appearance of additional reflections, so called satellite reflections, was already noted in the forties in some alloys (Daniel and Lipson, 1944;Hargreaves, 1951) and later in the silicate mineral labradorite. Korekawa (1967) presented a theory of satellite reflections relating their presence to some specific deformation and in particular modulations of the crystalline structure.…”